By Attribution

Flickr lets each user easily choose under which license they
want to publish their work:

None (All rights reserved)

Attribution-NonCommercial-ShareAlike Creative Commons

Attribution-NonCommercial Creative Commons

Attribution-NonCommercial-NoDerivs Creative Commons

Attribution Creative Commons

Attribution-ShareAlike Creative Commons

Attribution-NoDerivs Creative Commons

Say, you would like to make your photos to be available as freely as
possible - make derivative works, use commercially, whatever. The
most permissive of these licenses is the Creative Commons Attribution
license, which only requires that the users of your work attribute
you.

You must attribute the work in the manner specified by the author or
licensor.

Some Flickr users do specify how to attribute, but most of them don't,
and that's perfectly ok, as not all of them are lawyers. But still,
how do you attribute when the author hasn't specified how. Buried
deep inside the legal text you find that the license does in
fact say how one should attribute:

You must [--] keep intact all copyright notices for the Work and
provide [--]:

(i) the name of the Original Author (or pseudonym, if applicable)
if supplied [--]

(ii) the title of the Work if supplied;

(iii) to the extent reasonably practicable, the URI, if any, that
Licensor specifies to be associated with the Work [--]

(iv) [--] in the case of an Adaptation, a credit identifying the use
of the Work in the Adaptation (e.g., "French translation of the Work
by Original Author," or "Screenplay based on original Work by
Original Author"). [--]

Note that I have heavily edited the above text as the original blub of
legalese is almost impossible to read. Luckily it all follows a
common sense.

It's easy to attribute when you just use or adapt a single work. It's
not particularly hard to attribute 3, 5 or even 10 works. But what if
you want to use hundreds of works? Say, you would like to create a
photographic mosaic, that kind of work needs thousands of
photos to choose from and only few hundreds of those photos get chosen
by the program. In that case attributing becomes a far greater
trouble than the work required to create the mosaic at the first
place.

I'm sure many people would happlily give their photos away without not
even attribution required, but Flickr doesn't provide that option.